1. Field Of The Invention
This invention relates to a system for controlling the operation of a remotely controlled motor operated valve to control the force applied to the valve through the motor during the seating and backseating of the valve. More particularly, this invention relates to a system for eliminating, reducing or detecting packing leaks in a motor operated valve in an inaccessible location by electrically backseating the valve in a manner which does not pose a risk of damage to the components of the valve from the backseating operation. Additionally, when normal controls fail and render the valve inoperable this invention provides a substitute control system which can be aligned and selected to emulate normal controls thereby preventing damage to the components of the valve from a normal seating operation. Finally, the invention provides accurate recording of motor current signature for diagnostic purposes.
2. Description Of The Prior Art
Remotely controlled motor operated valves are widely used in fluid handling systems in nuclear power plants and other types of industrial plants where the valves may be inaccessible to plant personnel during the operation of the plant due to a hazardous condition which may prevail at the location of the valve, for example, a nuclear radiation environment which may prevail in a nuclear power plant containment building where many valves of this type are used. A similar problem may be encountered with respect to a motor operated valve in an electric power plant operated on fossil fuels due to the possibility of a steam leak in the environment of such valve, or with respect to a motor operated valve in a chemical plant due to the toxic nature of various fluids which are handled in such plants.
Typical valves which are used in fluid handling systems, for example, gate valves and globe valves, are subject to leakage at the packing around the movable stem that carries the gate or globe valve member, and it is known that such leakage can frequently be corrected, or at least substantially reduced, by backseating the valve, that is, by driving the valve past its normally opened position, where it is normally stopped by a limit switch in the valve actuator, so that it seats or nearly seats against a stop surface, called a backseat, at its maximum travel position. However, systems heretofore employed in the electrical backseating of a motor operated valve in an inaccessible location, for example, a system for bypassing the limit switch at the valve open position and then driving the valve toward its backseat until the locked rotor current of the valve motor is approached, are systems which are not reproducible and which pose the risk of damage to various components of the valve, such as valve stem failure, valve stem elongation, backseat damage, cracking of the stem nut, and damage to other valve components. These problems are extensively discussed in an Information Notice of the United States Nuclear Regulatory Commission, Office Of Nuclear Reactor Regulation, NRC Information Notice No. 87-40, dated Aug. 31, 1987 and entitled "Backseating Valves Routinely To Prevent Packing Leakage", and arise due to the relatively short period of time that elapses between the time that the motor current starts to increase as a result of the increasing torque which results from backseating, and the time when the valve is fully backseated. Further, since the system described in the aforesaid NRC Information Notice relies upon an operator to manually interrupt the motor current upon noticing an increase in the magnitude thereof, the system is not accurately reproducible from cycle to cycle of the same valve.
The Nuclear Regulatory Commission has emphasized other problems related to motor operated valves in an information bulletin of The United States Nuclear Regulatory Commission, Office of Inspection and Enforcement, NRC IE Bulletin #85-03, dated Nov. 15, 1985 and entitled "Motor-Operated Valve Common Mode Failures During Plant Transients Due To Improper Switch Settings." This bulletin discusses various problems with motor operated valve control system settings and sets forth a requirement that all licensees review and maintain all switch settings to ensure proper operation of safety related motor operated valves.